1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode having a barrier layer with a superlattice structure.
2. Discussion of the Background
In general, since Group-III-element nitrides, such as GaN, AlN, InGaN and the like, have an excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been considerably noticed due to its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high-density light sources, high-resolution output systems and optical communications.
FIG. 1 is a sectional view illustrating a conventional LED.
Referring to FIG. 1, the LED comprises an N-type semiconductor layer 17, a P-type semiconductor layer 21 and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 21. The N-type and P-type semiconductor layers 17 and 21 are formed of Group-III-element nitride semiconductors, i.e., (Al, In, Ga)N-based compound semiconductors. Meanwhile, the active region 19 is formed to have a single quantum well structure having a single well layer, or a multiple quantum well structure having a plurality of well layers, as shown in this figure. The active region 19 with a multiple quantum well structure is formed by alternately laminating InGaN well layers 19a and GaN barrier layers 19b. The well layers 19a are formed of semiconductor materials with a narrower band gap than the N-type and P-type semiconductor layers 17 and 21 and the barrier layers 19b, thereby providing quantum wells in which electrons and holes are recombined with each other.
Such a Group-III-element nitride semiconductor layer is grown on a different-type substrate 11 with a hexagonal structure, such as sapphire or SiC, using a method, such as metal organic chemical vapor deposition (MOCVD). However, if a Group-III-element nitride semiconductor layer is grown on the different-type substrate 11, a crack or warpage occurs in the semiconductor layer and dislocations are produced due to the difference of lattice constants and thermal expansion coefficients between the semiconductor layer and the substrate.
In order to prevent these problems, a buffer layer is formed on the substrate 11. The buffer layer generally includes a low-temperature buffer layer 13 and a high-temperature buffer layer 15. The low-temperature buffer layer 13 is generally formed of AlxGa1-xN(0≦x≦1) at a temperature of 400 to 800° C. using a method, such as MOCVD. The high-temperature buffer layer 15 is then formed on the low-temperature buffer layer 13. The high-temperature buffer layer 15 is formed of a GaN layer at a temperature of 900 to 1200° C. Accordingly, crystal defects in the N-type GaN layer 17, the active region 19 and the P-type GaN layer 21 can be considerably removed.
However, although the buffer layers 13 and 15 are employed, crystal defect density in the active region 19 is still high. Particularly, in order to enhance a recombination rate of electrons and holes, the active region 19 is formed to have a semiconductor layer with a narrower band gap than the N-type and P-type GaN layers 17 and 21. In addition, the well layer 19a is formed of a semiconductor layer with a narrower band gap than the barrier layer 19b. The semiconductor layer with a narrow band gap generally contains a large amount of In and thus has a large lattice constant. Therefore, lattice mismatch occurs between the well layer 19a and the barrier layer 19b and between the well layer 19a and the N-type semiconductor layer 17. Such lattice mismatch between the layers causes pin holes, surface roughness and degradation of crystal structures.